Following the publication of two landmark randomized trials, docetaxel chemotherapy became the standard of care for men with metastatic castrate-resistant prostate cancer (mCRPC). Prior to 2010, docetaxel/prednisone was the only treatment which had demonstrated a survival benefit in patients with mCRPC. Recent advances in the treatment of mCRPC have revolutionized treatment algorithms. Despite their impact on overall survival (OS), sipuleucel-t and alpharadin have unknown impact in symptomatic patients or those with visceral metastasis, respectively. Abiraterone and enzalutamide have favorable toxicity profiles, however they share mechanisms of resistance that likely diminish the benefits of sequential use. Thus, the benefit of these treatment for mCRPC remains limited. We investigated the selectivity and efficacy of new docetaxel-based combinations and novel molecular agents for CRPC in both the preclinical and clinical setting. Trebananib (AMG386) is a novel peptide-Fc fusion protein that sequesters angiopoeitin 1/2, thereby preventing the interaction with their common receptor Tie2, and inhibiting cell proliferation and tumor growth. Previous studies have demonstrated that in vivo alterations of testosterone levels regulate the expression of vascular endothelial growth factor (VEGF), fibroblast growth factor, and angiopoietin family members. Thus, dual targeting of the androgen and angiogenic axis represents a novel approach as a potential targeted therapy for patients with mCRPC. In collaboration with Dr. Bill Dahut (GMB, CCR, NCI), we initiated a randomized, phase II (with a run-in phase of the combination) trial of abiraterone/prednisone with or without trebananib in patients with mCRPC. Trebananib in combination with abiraterone is well tolerated, displayed an acceptable safety profile in the run-in phase of the trial, and established 30 mg/kg as the phase II dose in combination with abiraterone. We are also conducting the angiogenesis biomarker and PG analyses in this trial. Analyses of the trial data is ongoing. Cabozantinib is a small molecule inhibitor of multiple tyrosine kinases including MET, VEGFRs and RET. It was developed as an inhibitor of both angiogenesis and of its resistance mechanism. We hypothesized that the combination of docetaxel with cabozantinib could potentially minimize tumor resistance by targeting different cellular signaling pathways without overlapping toxicities. In collaboration with Dr. Dahut, we conducted a phase I/II study to determine the safety profile of cabozantinib in combination with docetaxel and prednisone, and to determine the MTD as the recommended phase II dose in combination with docetaxel in mCRPC patients. The phase I accrued 19 patients and established the MTD of cabozantinib at 40mg daily with docetaxel. For 13 patients previously treated with abiraterone in phase I, the median potential follow up is 12.4 months and 9 month PFS is 60.5%. Docetaxel plus prednisone may have limited benefits after disease progression on abiraterone as seen in three retrospective analyses demonstrating a median PFS of 4.6 months or less. PFS results seen in this trial compare favorably to historical data of treatment with docetaxel after progression on abiraterone, suggesting the addition of cabozantinib to docetaxel may help overcome acquired resistance. A potential drug-interaction between docetaxel and cabozantanib was based on prior knowledge that both drugs are metabolized by CYP3A4 and that cabozantanib is also a weak competitive inhibitor of CYP3A4 (Kiapp=282 uM; or 141 ug/mL). Docetaxel concentration-time curves did not appear to be substantially different in the absence or presence of cabozantanib. Preliminary PK data showed no significant differences in any PK parameter alone or with cabozantinib. Analyses of trial data is ongoing. We are also currently analyzing clinical outcome data from this trial versus genotypes obtained via the DMET array and several angiogenesis pathway polymorphisms. Our understanding of the biology of mCRPC progression has led to the discovery of more effective targeted approaches that involve modulation of the androgen-AR system. Recognition of the anti-androgen withdrawal response resulted in the development of therapy targeting androgen production (anti-androgens) or AR-mediated signaling (AR antagonists). Seviteronel (VT-464) is a novel, non-steroidal, small-molecule CYP17A1 inhibitor with 17,20-lyase selectivity. We are involved in the preclinical and clinical development of this compound. Preclinical results suggest greater suppression of the AR axis with VT-464 than abiraterone that is likely due to both superior selective suppression of androgen synthesis and AR antagonism. In collaboration with Dr. Ravi Madan (GMB, CCR, NCI), two phase II trials of VT-464 have recently been initiated at the NCI in CRPC patients previously treated with enzalutamide (BID dosing) or progressing on enzalutamide or abiraterone (QD dosing). Both trials are currently open to accrual and we are performing the PK and PG analyses for these trials. We are also interested in understanding the mechanisms of resistance of prostate cancer regimens. Enzalutamide is a potent second-generation androgen receptor (AR) antagonist with activity in CRPC. Although enzalutamide is initially effective, disease progression inevitably ensues with the emergence of resistance. Intratumoral hypoxia is also associated with CRPC progression and treatment resistance. Given that both AR and hypoxia inducible factor-1alpha (HIF-1a) are key regulators of these processes, dual targeting of both signaling axes represents an attractive therapeutic approach. Crosstalk of the AR and HIF-1a signaling pathways were examined in prostate cancer cell lines (LNCaP, 22Rv1) with assays measuring the effect of androgen and hypoxia on AR-dependent and hypoxia-inducible gene transcription, protein expression, cell proliferation, and apoptosis. HIF-1a inhibition was achieved by siRNA silencing HIF-1a or via chetomin, a disruptor of HIF-1a-p300 interactions. In prostate cancer cells, the gene expression of AR targets (KLK3, FKBP5, TMPRSS2) was repressed by HIF-signaling; conversely, specific HIF-1a target expression was induced by dihydrotestosterone-mediated AR signaling. Treatment of CRPC cells with enzalutamide or HIF-1a inhibition attenuated AR-regulated and HIF-1a-mediated gene transcription. The combination treatment was more effective than either treatment alone. Similarly, the combination also reduced VEGF protein levels. HIF-1a siRNA synergistically enhanced the inhibitory effect of enzalutamide on cell growth in LNCaP and enzalutamide-resistant 22Rv1 cells via increased enzalutamide-induced apoptosis. In conclusion, the combination of enzalutamide with HIF-1a inhibition resulted in synergistic inhibition of AR-dependent and gene-specific HIF-dependent expression and prostate cancer cell growth. Taken together, dual targeting of the AR and HIF-1 axis represents an attractive approach for CRPC treatment with HIF-1a inhibition as a possible mechanism for overcoming enzalutamide resistance and potentiating anti-AR therapy. Enzalutamide's effect on HIF-1a-regulated gene expression is specifically targeted to VEGF, thus making VEGF a potential biomarker for assessing enzalutamide response; studies are underway to evaluate this effect in clinical treatment samples. In addition, we are also involved in developing several specific small-molecule HIF-1a inhibitors, and ongoing preclinical studies are evaluating the combination of these HIF-1a inhibitors with enzalutamide in prostate cancer xenografts. The goal is to identify a potential HIF-1a inhibitor candidate to move forward in a phase I trial in combination with enzalutamide for patients with mCRPC.